Ultrasound diagnostic apparatuses that measure, for example, the blood flow velocity of a subject by using the Doppler method are widely employed. Such ultrasound diagnostic apparatuses are, for example, continuous wave Doppler devices, pulse-Doppler devices, etc. The continuous wave Doppler device transmits an ultrasonic wave as a continuous wave to a subject. Thereafter, the device sequentially receives the ultrasonic waves reflected from the subject along the time axis, and employs the Doppler shift frequency components of the ultrasonic waves to display the Doppler waveforms that represent a temporal change. The pulse-Doppler device transmits a pulse-modulated ultrasonic wave to a subject at a predetermined time interval. Thereafter, the device receives, at the timing consonant with the depth of the region to be examined, the ultrasonic waves reflected from the subject, and employs the Doppler shift frequency components of the ultrasonic waves to display the Doppler waveforms.
For the pulse-Doppler device, the velocity to be measured is limited in accordance with the time interval of transmission of the pulse modulated wave. On the other hand, there is no such limitation for the continuous wave Doppler device, and the upper limit of the velocity to be measured is higher than that for the other devices that employ the Doppler method. Therefore, the continuous wave Doppler device is frequently employed to rapidly diagnose the presence/absence of blood flow due to a circulatory system disease.
Generally, noise may appear on the image obtained by the ultrasound diagnostic apparatus due to external noise, thermal noise, non-linearity of the circuitry, etc., and may degrade the visibility of the image. Therefore, a technique for improving the visibility of the image obtained by the ultrasound diagnostic apparatus has been proposed. For example, in patent documents 1 to 3, a technique is described for enhancing the contour of an image obtained by the ultrasound diagnostic apparatus. The ultrasound diagnostic apparatus described in these patent documents calculates the mean value and the variance of pixel values for a group of pixels in a predetermined area that includes a target pixel. Thereafter, the pixel value of the target pixel is changed based on differences between the value of the target pixel and the mean and the variance thus obtained, and the visibility of the image is increased.
Further, an ultrasonic Doppler diagnostic apparatus is described in patent document 4. This apparatus obtains, as a data sample area, an area of a Doppler waveform image located at a predetermined distance from the base line that indicates a velocity of zero, and employs the luminance distribution for the individual pixels in this area to detect a reference noise level. Thereafter, the reference noise level is employed to perform automatic tracing for the Doppler waveform.